1. Introduction

2. Results

2.1.1. Precursors investigation

Thermal analyses of dried precursors were performed in order to establish the optimum crystallization temperature for the corresponding TiO₂ oxides. Figure 1 show the thermogravimetric (TG) and differential thermal analysis (DTA) and derivative thermogravimetric (DTG) curves of dried (at room temperature, for 10 days) titanium precursor’s solution in the temperature range 20°C – 1200°C at a heating rate of 10°C/min. in an airflow.

Considering that the quantity of Ag doping does not have a significant influence on the thermal transformations and the total mass loss of samples, we chose for the thermal investigation the sample with the highest percentage of Ag in the TiO₂ matrix (Ag⁺ /Ti⁴⁺ = 2.5%).

Figure 1. Thermogravimetric(TG), differential thermal analysis (DTA) derivative thermogravimetric (DTG) curves of the dried (100°C) titania precursors (Ag⁺ /Ti⁴⁺ = 2.5%).

Figure 1. Thermogravimetric(TG), differential thermal analysis (DTA) derivative thermogravimetric (DTG) curves of the dried (100°C) titania precursors (Ag⁺ /Ti⁴⁺ = 2.5%).

The thermal decomposition of TiO₂ precursors take place in different progressive steps of temperature (TG) depending on physical and chemical processes that occur with increasing the temperature. The total weight loss (44,75%) associated to transformation of precursors into Ag⁺ / Ti⁴⁺ = 2.5% is the results of physical and chemical adsorbed water (8.07%), volatilization and combustion of residual water and organic parts from polymeric gels (20.63%gr), combustions of residual carbon (6.45%), Ag-crystallization of amorphous anatase (9.6%). The corresponding thermal effects are shown in DTA curve. Thus, a significant effects corresponding to the exothermic peak at ~ 420.43°C in the DTA curve are related to the beginning of crystallization of TiO₂ anatas, whereas that around 543.57°C indicates the phase transformation anatase – rutile. These transformations are confirmed by the transformations corresponding to the DTG curves.

To further investigate the TiO₂ precursors, the FT-IR spectra of the precursor solution containing Ag⁺ / Ti⁴⁺ = 2.5% are presented in Figure 2. The absorption shoulders at 1634cm^-1 is attributed to bending vibrations of the –OH groups, and the bands centered at 1580, 1530, 1430 and 1025cm^-1 corresponded to conjugated C – O vibrations [C = C - (C = O)] and [C = C - (C-) O-] of the acac ligand. The peak at 1355 and 931cm^-1 is related to the CH₂ symmetric vibration and CH₃ – C – CH₃ stretching modes of the isopropoxy group. The band at 1126cm^-1 is due to Ti-O-C stretching vibration, the broad peaks in the range 653 – 550cm^-1 and 495 - 436cm^-1 indicated the presence of ν_Ti-O and ν_Ti-O-Ti bond respectively, and the peak located at 443cm^-1 is correlated with asymmetric stretching vibration of Ti-O-Ag [21].

Figure 2. The FT - IR spectra of precursor solution containing Ag⁺/Ti⁴⁺ = 2.5%.

Figure 2. The FT - IR spectra of precursor solution containing Ag⁺/Ti⁴⁺ = 2.5%.

2.2. Structural, morphologic and optical properties

Crystalline structure and structural parameters of MWCNT-TiO₂-Ag nanocomposites were confirmed by XRD as shown in figure 3a. The indexed diffraction peaks correspond to tetragonal anatase crystalline phase (PDF card 01-089-4921) and Ag (PDF card 00-002-1098). No other peaks related to secondary phases was observed in the diffractograms. Devi et al. [22] reported that Ag deposited nitrogen doped TiO₂ lead to the same effects. The peak at la 2θ=26.4° is typical for the 002 direction of graphite [23] (file PDF 01-075-0444) is probably superposed with (101) at 25.38° of anatase [24,25]. The small right - shifting to higher 2θ values of diffraction peaks of anatase (101) with the increse in Ag content (Figure 3b) less than 0.1°, was reported by Yuliati et al. [26] and was assigned to the presence of Ag on TiO₂ with minimal crystal distorsion. The presence of MWCNT is also underlined by the elemental chemical analysis (Figure 3).

Figure 3. XRD diffraction patterns of MWCNT - TiO₂ - Ag nanocomposites. a. 2θ = 20 - 80° and b. 2θ = 24.8 - 26°.

Figure 3. XRD diffraction patterns of MWCNT - TiO₂ - Ag nanocomposites. a. 2θ = 20 - 80° and b. 2θ = 24.8 - 26°.

The Warren - Averbach X-ray profile Fourier analysis of the (101) (2θ=28.31°), (200) (2θ= 48.05) and (215) (2θ=75.058°) anatase peak profiles were processed by the XRLINE [27] computer program in order to determine the effective crystallite mean size (D_eff). The crystallite size distribution function was determined from the second derivative of the strain corrected Fourier coefficients [28]. Also, from this profile were obtained the root mean square (rms) of the microstrains averaged along the real space distance, ${\langle {\epsilon }^2\rangle }_{hkl}^{1/2}$ [29].

The elementary parameters of the cell was determined by Rietveld type refinement using the Powder Cell program [30], developed by Werner Kraus & Gert Nolze (BAM Berlin).

Table 1 summarizes the microstructural parameters of TiO₂ anatase nanoparticles from the MWCNT-TiO₂-Ag.

Table 1. The effective crystalline mean size, ${\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}$(nm), the root mean square (rms) of the microstrains averaged along the real space, ${\langle {\mathsf{\epsilon }}^2\rangle }_{\mathrm{h}\mathrm{k}\mathrm{l}}^{1/2}.$

Table 1. The effective crystalline mean size, ${\mathrm{D}}_{\mathrm{e}\mathrm{f}\mathrm{f}}$(nm), the root mean square (rms) of the microstrains averaged along the real space, ${\langle {\mathsf{\epsilon }}^2\rangle }_{\mathrm{h}\mathrm{k}\mathrm{l}}^{1/2}.$

Sample	Unit cell parameter		Cell volume [Å3]	$\mathrm{Effective} \mathrm{crystalline} \mathrm{mean} \mathrm{size}, {\mathit{D}}_{\mathit{e}\mathit{f}\mathit{f}}$(nm)	$\mathrm{Microstrains} \mathrm{averaged} \mathrm{along} \mathrm{the} \mathrm{real} \mathrm{space}, {\langle {\mathit{\epsilon }}^2\rangle }_{\mathit{h}\mathit{k}\mathit{l}}^{1/2}$x 103
	a [Å]	c [Å]
MWCNT - Ag+/ Ti4+ = 0.5%	3.7717	9.4603	134.576	14.21	11.3
MWCNT - Ag+/ Ti4+ = 1.0%	3.7824	9.4821	135.656	14.08	12.44
MWCNT - Ag+/ Ti4+ = 1.5%	3.7892	9.4888	136.240	13.86	13.86
MWCNT - Ag+/ Ti4+ = 2.0%	3.7920	9.4842	136.375	13.35	14.2
MWCNT - Ag+/ Ti4 += 2.5%	3.7951	9.5046	136.892	13.18	15.6

From microstructural data, it can be observed that as the dopant concentration increases the average crystallite decreases from 14.21 to 13.18nm.The decrease of crystallite size is due to the effects of Ag at the boundary of TiO₂ [31].

2.3. FT – IR spectroscopy

Figure 4 presents the FT-IR spectra obtained for MWCNT - TiO₂-Ag nanocomposites with various Ag⁺/Ti⁴⁺ ratios. In the FT- IR spectra, the peak from 3414 cm^-1 can be associated to the -OH stretching vibrations. The C-H stretching mode from the MWCNT structure [32] are evidenced in all the MWCNT - TiO₂ - Ag samples at about 2852 and 2924cm^-1 (figure 4.). The bending mode of the adsorbed water molecules on the nanocomposites are responsible for the 1637 cm^-1 peak [33]. The band associated with Ti-O-Ti lattice vibration at 1384cm^-1 can be observed [33]. The broad band centered at 480 cm^-1 and 548 cm^-1 could be associated with the Ti-O-C stretching vibrations and to Ti–O vibration, respectively [34]. In the spectrum of MWCNT-Ag⁺/Ti⁴⁺= 0.5%, more bands can be identified and associated with the presence of the COOH (peak from 1744 cm^-1) or C-O-C vibrations (peak from 1084 cm^-1) [34,35]. They can be also seen in less intensity in the other FTIR spectra of MWCNT- TiO₂ - Ag samples, suggesting the presence of the MWCNT structures in the composition of all these hybrid materials.

Figure 4. The FT- IR spectra of MWCNT- TiO₂ -Ag.

Figure 4. The FT- IR spectra of MWCNT- TiO₂ -Ag.

2.4. Raman spectroscopy

The typical Raman active modes of anatase TiO₂ are composed of four main peaks around 152 (E_g), 389 (B_1g), 510 (A₁ ) and 633 cm^-1 (E_g ). In addition to these bands, a small peak, assigned to the E_g mode, is localized at about 189 cm^-1 [36]. The MWCNT - TiO₂ - Ag nanocomposites contain supplementary bands which are characteristic to the carbon nanostructures, Figure 5. Thus, a D band appeared at about 1361 cm^-1, being specific to the presence of defects in the carbon system, while the G peak from about 1589 cm^-1 is related to the vibration of the sp² – bonded carbon atoms [37]. A 2D band (also called G’) is present at about 2712 cm^-1 in the spectrum of the MWCNT-Ag⁺/Ti⁴⁺ =2.5% sample and at 2718 cm^-1 in the CNT precursor sample.

Figure 5. Raman spectra of MWCNTs – TiO₂ with different amounts of Ag nanoparticles recorded with 514 nm excitation laser line.

Figure 5. Raman spectra of MWCNTs – TiO₂ with different amounts of Ag nanoparticles recorded with 514 nm excitation laser line.

2.5. Electron spin resonance measurements

EPR spectra are shown in the Figure. The analysis of EPR spectra reveal a narrow resonance signal centered at g = 2.004 due to an oxygen vacancy with trapped electron named F - center [38]. By increasing the Ag content in TiO₂ lattice, this signal gradually diminishes. Besides this signal, at higher and lower field side there are two low resonance signal centered at g = 1.97 and g = 2.03 shown in the inset of the Figure 6. The signal at g = 1.97 was attributed to Ti³⁺ ion in anatase crystalline phase, and the signal at g = 2.03 show the presence of surface O^- ions [38].

Figure 6. a. EPR spectra of MWCNT - TiO₂ - Ag.

Figure 6. a. EPR spectra of MWCNT - TiO₂ - Ag.

The EPR integral intensity of the resonance signal, which is proportional with the number of the electron spins from sample is illustrated in Figure 6b. It could be observed that all resonance signals show same behavior, decreasing with the increase of dopant concentration up to 1.5 % mol Ag, followed by a slightly increase. The oxygen vacancies concentration shows a significant decrease for sample with high Ag content (≥1.5 mol% ).

2.6. UV – ViS spectroscopy

Figure 7 indicates the UV – VIS spectra of MWCNTs – TiO₂ with different Ag nanoparticles amounts. An intense absorption is observed at 353 nm specific to intrinsic band gap of TiO₂ nanoparticles [39]. The broad absorption which covers the whole range of visible range, is caused by the addition of MWCNTs [40]. Moreover, the strong absorption peak around 470 – 580nm arise from the surface plasmon absorption due to Ag particles [41]. By increasing the Ag concentration a red shift to a higher wavelength is observed in the absorption edge of the nanocomposites, due to electronic interaction between MWCNT and TiO₂ modified Ag [42] . Supplementary, the spectrum contain a very low absorption peak at 263 nm attributes to the transition to Ag higher energy state of valence electrons [43].

Figure 7. UV – Figure 7. UV - VIS spectrum of MWCNTs – TiO₂ -Ag.

Figure 7. UV – Figure 7. UV - VIS spectrum of MWCNTs – TiO₂ -Ag.

2.7. Optical band gap energy, E_g determination

The E_g of the nanopowder was estimated by using Tauc's plot [44,45]. The band gap values for direct transitions is 2.81-3.02eV (Figure 8) (Table 2). The redshift to the longer wavelengths originates, as we mention and was obsreved, aslo, by Li et al. [46], from the band gap narrowing.

Figure 8. Extrapolation of the band gap energy for direct transition.

Figure 8. Extrapolation of the band gap energy for direct transition.

Table 2. Optical band gap energy of MWCNT -TiO₂-Ag nanocomposites.

Table 2. Optical band gap energy of MWCNT -TiO₂-Ag nanocomposites.

Sample	Eg – direct transition
MWCNT - Ag+/ Ti4+ = 0.5%	2.77
MWCNT - Ag+ / Ti4+ = 1.0%	2.73
MWCNT - Ag+ / Ti4+ = 1.5%	2.69
MWCNT - Ag+ / Ti4+ = 2.0%	2.46
MWCNT - Ag+ / Ti4+ = 2.5%	2.41

2.8. Morphology of nanocomposites

Conventional TEM images revealed the presence of polyhedral nanopartices over the nanotube surface having a tendency to agglomerate.

Figure 9. TEM images of MWCNTs - TiO₂ - Ag . a. Ag⁺ /Ti⁴⁺ = 0.5% and b. Ag⁺ /Ti⁴⁺ = 2.5%.

Figure 9. TEM images of MWCNTs - TiO₂ - Ag . a. Ag⁺ /Ti⁴⁺ = 0.5% and b. Ag⁺ /Ti⁴⁺ = 2.5%.

In addition, the EDX shown in Figure 10 reveals the presence of elements including C, Ti, O and Ag in the MWCNTs –TiO₂ – Ag samples.

Figure 10. EDX mapping of MWCNTs – Ag⁺/Ti ⁴⁺ = 1.5%.

Figure 10. EDX mapping of MWCNTs – Ag⁺/Ti ⁴⁺ = 1.5%.

2.11. Photoluminescence spectroscopy.

The PL spectra of MWCNTs – TiO₂ with different amount of Ag nanoparticles is illustrated in Figure 11. As can be seen in the PL spectra, the intensity of the PL bands decreases with the incorporation and increasing of the content of Ag⁺ ions in the TiO₂ the content of Ag⁺ ions in the TiO₂ lattice, because Ag nanoparticles act as trapes to capture the photoreduced electrons and thus inhibit recombination of electron – hole pair. Presence of peak at ~ 470 nm is due defect states formed by oxygen vacancies [47], and peak from 330nm is associated with band – gap of titania [48]. Emissions around ~550 nm are associated to electron transition from $\left[{TiO}_6\right]$to O on the surface [49].

Figure 11. PL of MWCNTs – TiO₂ – Ag nanocomposites.

Figure 11. PL of MWCNTs – TiO₂ – Ag nanocomposites.

2.12. Photocatalytic activity.

Photocatalytic activity of the samples was investigated using a synthetic solution of Allura red-E129 under UV irradiation. Since the efficiency of the degradation process depends on both the concentration of the pollutants and the amount of the photocatalyst in the studied systems, minimal amounts of each component were chosen for the study.Prior to irradiation, the samples were kept in dark for 60 min to achieve adsorption-desorption equilibrium. The testing of the photocatalytic activity of MWCNT decorated with TiO₂-Ag nanoparticles was carried out by tracking the spectrophotometric decrease, over time, of the concentration of pigment E129, $\left(c_{0 }=2 · {10}^{-5}\right)$(λ = 502nm), following adsorption and photodecomposition of nanostructures. Figure 12 shows degradation efficiency of the samples. The adsorption capacity of the samples increases with Ag content.

The efficiency of the photocatalysts for selected Allura Red degradation was calculated according to Eq. (1):

$$F\left(\%\right)= \frac{C_0- C_t}{{C }_0}x 100$$

(1)

where Co is the initial concentration of dye, and Ct is the concentration of colorant at time t. The nanostructures with 2.0% Ag have the best photocatalytic activity for the degradation of Allura Red under UV irradiation demonstrating degradation efficiency of 79.99% (Figure 12) .According to literature data the photocatalytic activity of Ag doped titania is dependent on the concentration of noble metal particles [50,51] and the diminish of the photoreaction at higher loading is due to so-called “screening effect” [52]. The presence of Ag nanoparticles stimulates the separation of charges but too high concentration of Ag particles shields the active centers of the semiconductor, thus reducing its photocatalytic activity.

Figure 12. Photocatalytic degradation of Allura Red by MWCNT nanostructures.

Figure 12. Photocatalytic degradation of Allura Red by MWCNT nanostructures.

According to Langmuir – Hinselwood expression:

$-ln \left(\frac{C_t}{C_o}\right) =kt$, unde $C_0$ and $C_t$ are the dye concentration at time 0 and t (after irradiation), k (min^-1) is the pseudo - first order rate constant, the best photocatalyst is the one with 2.0% Ag (Figure 13), as mentioned above. The obtained results are shown in Table 3.

Figure 13. Kinetics of Allura Red degradation by nanostructures.

Figure 13. Kinetics of Allura Red degradation by nanostructures.

Table 3. Photodegradation rate, apparent first-order rate constant (k_i) of photocatalytic degradation and correlation coefficient (R²).

Table 3. Photodegradation rate, apparent first-order rate constant (k_i) of photocatalytic degradation and correlation coefficient (R²).

Sample	Photodegradation rate (%)	$k_i · {10}^{-3}$	R2
MWCNT - Ag+/ Ti4+ = 0.5%	47.5	3. 06	0.95097
MWCNT - Ag+/ Ti4+ = 1.0%	65	5.30	0.91623
MWCNT - Ag+/ Ti4+ = 1.5%	55	3.57	0.97206
MWCNT - Ag+/ Ti4+ = 2.0%	80	8.42	0.98006
MWCNT - Ag+/ Ti4 += 2.5%	72.5	4. 66	0.98538

To identify the reactive oxygen species involved in the photocatalytic process of MWCNT-TiO₂-Ag, ESR coupled with spin-trapping method was used. As spin trapping agent was used DMPO.

The experimental spectrum obtained after 30 min UV irradiation of DMPO-MWCNT-TiO₂-Ag solution is shown in Figure 15. The obtained spectrum is a complex one consisting of many resonance lines due to different spin adducts generated in the time of irradiation. To identify and extract the contribution of the spin adducts, the simulation of the spectrum was performed. The simulated spectrum represents the linear contribution of the following spin adducts: •DMPO-OCH₃ ( = 13.2 G, = 8.3 G, = 2.0 G, relative concentration 23.8%), •DMPO-O₂^- ( = 13.0 G, = 11.3 G, = 2.6 G, relative concentration 37.9%), •DMPO-OOH ( = 13.5 G, = 11.0 G, = 0.6 G, relative concentration 17.2%), nitroxide-like radical ( = 13.9 G, relative concentration 21%). The presence of •DMPO-OCH₃ spin adducts is the proof of the presence of •OH radicals as a results of the interaction between the DMSO solvent and •OH radical [53]. •OOH radical is obtained by protonation of superoxide radical •O₂^- [54]. Nitroxide-like radical appears by cleavage the N‒C bond and ring opening of DMPO [55]. These results attest the presence of both •OH and •O₂^- radicals, relative concentration•O₂^- radicals have been more than twice than hydroxyl radicals concentration.

Figure 15. Experimental and spectrum simulation of DMPO spin adducts generated by MWCNT-TiO₂-Ag sample after 30 min of irradiation.

Figure 15. Experimental and spectrum simulation of DMPO spin adducts generated by MWCNT-TiO₂-Ag sample after 30 min of irradiation.

4. Materials and Methods

5. Conclusions

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